A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. An actuator relies on suspension's force on the read/write head to provide the proper distance between the read/write head and the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away onto a mechanical landing ramp from the disk surface.
As the distance between the read/write head and the surface of the magnetic-recording disk must be tightly controlled, it is desirable that the magnetic-recording disks vibrate or oscillate as little as possible. To address this concern, a tied-shaft fluid dynamic bearings (FDB) spindle motor may be used to increase the rigidity of the motor structure and to minimize, and potentially eliminate, any vibration or oscillation of the magnetic-recording disks, especially when the HDD is bumped or otherwise experiences a mechanical shock.
FIG. 1 illustrates a tied-shaft fluid dynamic bearings spindle motor according to known approaches. FIG. 1 is taken from and discussed in U.S. Pat. No. 6,118,620. As illustrated in FIG. 1, upper symmetrical cone 10 and lower symmetrical cone 12 are symmetrical and serve to stabilize the magnetic-recording disk, both when magnetic-recording disk is rotating and when the HDD is bumped or otherwise experiences a mechanical shock. The dimensions of upper symmetrical cone 10 and lower symmetrical cone 12 are designed to be the same so that upper symmetrical cone 10 and lower symmetrical cone 12 have the same bearing stiffness. If the upper symmetrical cone 10 and lower symmetrical cone 12 have the same bearing stiffness, then it is expected that any force applied to the magnetic-record disk (perhaps due to the HDD being bumped) should be equally dampened by upper symmetrical cone 10 and lower symmetrical cone 12, thereby reducing or eliminating the tendency for the magnetic-recording disk to oscillate.